Laying back on the couch, with your girlfriend or best buds, enjoying a large greasy pizza and cold michelob light, while watching a good movie on the tube.

can be used as sarcasm for a terrible day, or series of events.

having a good meal, some good drinks, and getting laid on the same night. (when your favortie football team wins the super bowl)

stuff that goes on after marriage.

A good time is when you are describing an item, person or place. This isn't the usual meaning of a good time. This is when an item is a good time or that person is a good time. It's specifying a single thing that is a good time just in an odd way.
Same thing goes for "a bad time"